Fig. 4. Average ground reaction forces from two force plates during a single step of walking at three different step lengths (N=9). (A) Leading leg force plate, (B) trailing leg force plate. (C) Center of mass velocity for a single step, computed from ground reaction forces. The dot product of the ground reaction forces and center of mass velocity yields average external mechanical power produced by (D) the leading leg and (E) the trailing leg during a single step at different step lengths. As a result of the changes in ground reaction forces and center of mass velocities, the external mechanical power generated within a step increased at longer step lengths. Grey lines denote double support. Medio—lateral forces and velocities (not shown) are relatively small and change little with step length. They are, however, included in all calculations. l^*, preferred step length.

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